1. Field of the Invention
The present invention relates to electrodeposition painting devices and methods, and more particularly to electrodeposition painting devices and methods employing a first electrode serving as an article to be coated and a second electrode provided as a membrane electrode device, and including a membrane condition sensing capability.
2. Description of the Prior Art
In electrodeposition painting, there generally are techniques that use anion-type paints and techniques that use cation-type paints. With either of the cation-type or anion-type techniques, generally the uniformity and adherence of the paint film on the painted subject is superior and pollutants are limited over other types of painting technologies. As a result, recently there have been many widely used applications for such electrodeposition painting such as in automatic paint processing of automobile bodies, especially in the metal undercoating or the last coating, etc., of the automobile bodies.
Of the paints used in electrodeposition painting, for anion-type paints, an example is a resin of molecular weight 200 made water soluble by adhering to it a carboxyl group. For cation-type paints, an example is a similar resin made water soluble by adhering to it an amino group. However, the degree of ionization of these water soluble paints after dissolving in water typically is minute. For this reason, at present, when using anion-type paints an alkaline neutralizer such as tri-ethylamine is mixed with the paint, and when using cation-type paints an acetic neutralizer such as acetic acid is mixed with the paint, thereby obtaining neutralization and increasing the degree of ionization in the water.
According to the behavior of the resin component of each paint, a neutralizer is mixed in to increase the degree of ionization. As the electrodeposition coating of the painted article progresses, however, the resin component of the paint in the solution is consumed and therefor decreases, so that the paint in the solution must be replenished from the outside. At the same time, neutralizer is added by the replenishment of the paint. For this reason, as the electrodeposition of the painted article advances along with the consumption and replenishment of the paint, the amine and/or acetic acid neutralizer tends to continuously accumulate, which can result in the re-dissolving of the painted surface or in the generation of pinholes, causing the efficiency of the electrodeposition painting to fall to a significant degree.
In order to improve on such inconveniences and inefficiencies, recently, for example as can be seen in Japanese Patent Publication 45-22231, an ion exchange membrane, etc., used as a membrane for extracting neutralizer (hereafter referred to as "membrane") separates the article to be coated (which is one of the electrodes, namely the first electrode) and the aqueous solution from the other electrode, and through the membrane amine and/or acetic acid neutralizer is osmotically extracted from the aqueous solution, preventing an undesirable accumulation of the neutralizer in the aqueous solution, putting into practical effect a so-called pH control.
In the above-described present-day example, however, even if an ideal acid removal balance is attained, the electrical resistance of the membrane in use progressively increases (with continuous use of up to several years) to 10 to 50 times its initial value. If left in this higher resistance condition, not only does the efficiency of the acid removal fall, but inconveniently the efficiency of the electrodeposition painting significantly falls as well.
For example, for the sake of convenience, let the electrical resistance of the interior of the membrane electrode device be 1 .OMEGA. (ohm), the electrical resistance of the membrane be 1 .OMEGA., and the electrical resistance of the aqueous solution between the exterior of the membrane and the article to be painted be 8 .OMEGA.. Then, when the resistance of the membrane becomes 11 .OMEGA., the electrical current flowing through the solution is halved. As a result, the efficiency of the electrodeposition painting from the starting point effectively is already halved. When the resistance of the membrane becomes 31 .OMEGA., the current passing through the solution is decreased to 1/4 of its initial value. As a result, the efficiency of the electrodeposition painting from the starting point effectively is already decreased to 1/4 of the initial efficiency.